In a conventional screw machine, a male rotor and a female rotor, disposed in respective parallel overlapping bores defined within a rotor housing, coact to trap and compress volumes of gas. While such two rotor configurations are the most common design, screw machines are also known in the art having three, or more, rotors housed in respective overlapping bores so as to coact in pairs. Paired male and female rotors differ in their lobe profiles and in the number of lobes and flutes. For example, the female rotor may have six lobes separated by six flutes; the while conjugate male rotor may have five lobes separated by five flutes. Accordingly, each possible combination of lobe and flute coaction between the rotors occurs on a cyclic basis.
The rotors of a typical screw machine are mounted in bearings at each end so as to provide both radial and axial restraint. Nevertheless, in conventional practice, a certain amount of clearance in the axial direction must be provided between the end face of the rotors and the facing surface of the housing. The need to provide an end running clearance is primarily the result of thermal growth of the rotors as a result of gas being heated in the compression process. Maintaining the desired end running clearance at an amount sufficient to ensure that contact does not occur between the end face of the rotors and the facing surface of the housing is important to reliable operation of the screw machine. Additionally, during operation, the pressure gradient in the fluid being compressed normally acts on the rotors in an axial direction tending to force the rotors toward the suction end of the screw machine, thereby tending to increase the end running clearance.
If the end running clearance is too large, excessive circumferential and radial leakage of compressed fluid may occur through the running clearance at the discharge end of the screw machine thereby significantly decreasing the overall efficiency of the screw machine. In conventional oil-flooded screw machines, it is customary to supply oil to the interface zone defined by the end running clearance between the rotor end faces and the housing end plate as a means of providing a fluid seal to reduce gas leakage through the interface zone. However, as the end running clearance is reduced, efficiency losses due to viscous friction forces in the oil between the rotor end faces and the housing end plate tend to increase.
As noted previously, in operation the rotors grow in the axial direction toward the end casing at the discharge end of the housing due to thermal growth resulting from the fluid being heated in the compression process. This thermal growth of the rotors tends to reduce the end-running clearance. However, during operation the aforenoted axial pressure gradient tends to push the rotors in an axial direction towards the suction end of the screw machine, thereby tending to increase the end running clearance.
Therefore, in conventional oil-flooded screw machines, it is customary to maintain a substantial amount of end running clearance to minimize friction losses and, in the extreme, to prevent failure from rotor seizure. Such seizure may be a result of the thermal growth of the rotor due to the compression process. Also, as the end running clearance decreases, the viscous friction forces increase and may cause reduction in compressor operating efficiency.
As noted previously, the penalty for maintaining a large end running clearance is a consequent increase in leakage of compressed fluid. In order to maintain a large end running clearance in conventional oil-flooded screw compressors, it is known to add material to the end face of the rotors to provide a physical barrier to circumferential gas leakage. For example, elongated bar strips have been welded to rotor end faces so as to extend radially along the centerline of the lobes or lands of the rotors thereby extending across and bridging a substantial portion of the end-running clearance.